Method of solvent extraction



Nov. 26, 1946. K. H. HAcHMu'rH METHOD OF SOLVENT EXTRACTION med April 9, 194s ATTORNEYS.

. point of furfural.

Patented Nov. 26, 1946 v UNHELDl s TArEs METnonoF soLVEN'r EXTRACTION Karl H. Hachmuth, Bartlesvillekla.., assignor to Phillips Petroleum Company, a corporation of Delaware Application April 9,-1943,-seria'1 No. 482.435

claims.y (01. 2oz-39.5), j

` This invention relates toa method of usinga selective solvent for hydrocarbons, particularly:

aliphatic unsaturated hydrocarbons, wherein dry solvent is introduced at the top of column, water near the bottom ofthe column, and the hydrocarbon feed; at vsome-point intermediate to the introduction Aof the solvent and the bottom of the tower. More specifically,it relates to the use of frfural in the manner described for the recovery of butenes or butadiene. This method of using furfural permits utilization of the absorptive capacity of dry furfural and in the colder sections ofthe absorption tower it decreasesA the possibility of the formation of two liquid phases consistingof solvent and hydrocarbon; H i

Dry furfural has higher vabsorptive capacity yfor hydrocarbons than furiural vcontaining rsmall percentages of water. However, in the Stripping of dry rich furfural to remove the absorbed hydrocarbons, the temperature of operation is determined by the boiling points of the furfuralhydrocarbon mixtures at the given pressures. In

the case o f butene or butadiene recovery wherein a C4 fraction is selectively yextracted with f'urfural, it is desirable to maintain the pressure high enough in the stripping column that the thus.

recovered hydrocarbonsy canfbe readily condensed to a liquid by ordinary cooling means. -Conse fquently', the operating temperatures are necessarily relatively high. When small percentages of water are used, these temperatures areap- `preciably reduced and the resultant temperature reductions reflect the advantage of lowered furfural' and/or other polymer, gum, and coke formation, lowered heat loads, and less heat transfer equipment. Y f

The extent to which the incorporation of water in furfural effects a lowering ofthe boiling point varies with the amount of water used. The addition of up to 3cr 4 per cent by weight of water causes avery marked decrease in the boiling For additional small percentages the boiling pointis depressed considerably less. This is shown bythe following tabulation:

B-P. F.

Furfural Furfural+1% water by weight 296 Furfural-i-2% water by weight 229 Furfural' -i-3% Waterby Weight--. 220 Furfurali-4% water by weight 215 Furfural+6% water by weight 211 Furfural-i-8% water by weight 209.5

The boiling point approaches a minimum of 208 the absorption! F. It can be seen'that, ordinarily, little advantage is to be gained by using an amount of water lgreater than 4 to 8 per cent. The upper limit of Water added will usually be governed by the low- 5 est temperature in the absorbing and stripping system. For example, in a column using furfuralwater as a selective solventfor butadiene, operated at a pressure of 65 pounds per square inch absolutefthe kettle temperature is 250 F., while the temperatures near the top in the absorption sections are about 110" F. Therefore, it is not desirable to use more than 6.5 per cent yby weight of water in furfural, which is the limit above which two liquid phases would appear .at 110 F.

butadiene in furfural and The relative'volatility is defined as (garni/(raya)` where ya and :m are mol fractions -of anindividual hydrocarbon (in this case butenefl) in vapor and liquid, respectively, and' ya and :13B arethe mol fractions ofvbutadiene in vapor and liquid, respectively. y

selectivity rvof furfurl and furfural-wate'r fo butadiene over butehe-I;

fifty pounds per square inch gage pressure Y v v Relative Test No Solvent volatility y 1. 677 1. 75ok l. 814 1.683 1.618.

40 Average.. '....r.. l' 1.708 6 Furfural containing 8.9% water by weight... 1.847 7 (10.' 1.664.

l Average ..r 1 755 8 Furfural containing 4 water by weight l 694 9.-- .d0.. l l0.. l 11-- 1.725 i .A.ve1fag e .....v 1. 678.

As previously pointed out," the absorptive capacity of furfural for hydrocarbons is affected by the amount of water in the furfural while .the se lectivity is not appreciably changed. yThe absorptive capacity of furfural is decreased as -the amount of water added is increased. Consequently, there isla possibility for the separationor two The selectivity of'ifurfural for hydrocarbons is not greatly inuenced 4by small percentages water for'butadiene over butene-l, where theseby the relative volatility of.

liquid phases should any portion of an absorption system containing rich solvent and the proper amount of water operate at a relatively low temperature, such as is frequently obtained in the ytop part of an absorption column. The relative abby the following tabulation wherein is given Ithe maximum weight per cent of liquid n-butane,

Iuri'ural-water mixtures without the separation of two liquid phases. These data. are given for several different temperatures:

compositions are such to make likely separation of two liquid phases.

In the latter of the preceding tabulations, the solubility of C4 hydrocarbons in dry furfural and was shown. Reference to this table shows that the solubility of the hydrocarbons increases in the order of n-butane, butene-l, and butadiene. The volatility of these hydrocarbons in dry furfural or in furfural containing Water is in the reverse order of their solubility. That is, n-'butane is the most volatile, butene-l next most volatile, and butadiene the least volatile. Therefore, in the Solubility of liquid C4 hydrocarbons in liquid dry furfural and in liquid furfural containing water Furfursl (dry) Furfural+l water Furfural+3 a water Furfural+5% water: f

Qfantities given in per cent by weight. 1 atar phase separates.

I May be water phase separation.

This invention provides a means whereby the advantages of both dry furfural and furfural umn, and small quantities of water at some intermediate point in the column, preferably -below the point o! introduction of the hydrocarbon ly dry furl'ural in those sections of the column Where low temperatures and/or hydrocarbon Buta- N-bu- N bu- Bu- Buta diene tane tane tene-1 diene 10. 6 12. 9 m 9. 7 l1. 3 119.0 7.7 9.1 31.6 35.2 14.9 '7.3 8.7 l12.5 l12.9

absorbing tower the hyrocarbon of the least solubility will be present in the greatest concentration at `the top of the tower, Where the temperature is the lowest. As the hydrocarbon rich furfurai ilows water in the furfural without danger of separationor two liquid phases. When tolerance of the liquid `mixture for Water is great enough, the desired quantity of water may .be -added as steam or liquid.' Consequently, this arrangement reand Iurfural containing small percentages ot water.

Using furfural in the manner described makes it necessary to dry or partially dry the solvent before it is recrculatedto the absorption zone.

tures are therefore on the water-in-furfural side of the vapor-liquid boiling point composition vented to a compressor and returned to the rich furiural stripping column. In the case of recov- 'lby ordinary cooling means'.

ery of butadiene with furfural, these vent gases will be primarily butadiene and may be merged with the butadiene product efiluent from the stripper.

As previously mentioned, it is desirable to maintain the pressures in the absorption and stripping zones suiliciently high that the recovered hydrocarbon gases can readily be condensed The furfural fractionator, however, need not be run at such pressures. It may be run at substantially atmospheric or even under vacuum. Atmospheric presf sure or a slight positive pressure may be preferable in orderto insure keeping air out of the system. The oxygen of the air tends to ypromote furfural polymer formation. In general, the temperature in the bottom of the furriural fractionator under atmospheric pressure wl'il not be much greater thanthe temperature existing in' the re.

boiler section of the stripping zone containing furfural and water under the pressures required for the overhead gasproducts to be readily condensed by ordinary cooling means.

In most instances it is not necessary to completely dry the furfural. From a temperature standpoint, usually about 4 to` 8 weight per cent of water is desirable in furfural used as a selective solvent for hydrocarbons. Drying such mixtures to about one weight per cent watergreatly increases the absorptive capacity of the solvent, but it is necessary to dry the furfural only tothe extent'required to prevent two liquid phase separation under the conditions of operation of the absorption zone. Since this is true and since the pressures maintained in the absorption and strippingy zone' are usually relatively high,the furfural in many -cases may be sumciently dried by simply' flashing the lean vfurfural from the Y, (means not shown).

may

rconduit I3. conduit `I2 connecting with conduits II and I3 is provided for adding fresh vdry furfural from an' external sourcein order to permitrreplacing any lost from the system. The furfural- Water azeotrope is removed overhead from column I8 through conduit 5 and cooler I9 to enter accumulator 20. Cooler I9 is operated in such a way that the azeotropic mixture is` condensed and cooled sulciently that two liquid phases formv in accumulator 20. One phase, the lower one, is a furfural-rich phase which may be reiluxed in total or in part to column I8 through line I0. The other phase is a water-rich phase which is returned tol absorptionfcolumn I6 through conduits l 9 and I5. Conduit I4 connecting with 9 and I5 absorption and stripping systemk toy atmospheric pressure. If this is done, the furfural fractionator may be replaced with a ilash tank. The overhead and bottom products from the flashtank `may be handled in the same manner as with a fractionator, except that the furfural-rich phase resultlng from the condensing Aand cooling ofthe overhead product need not be reuxed to the flash tank, but may be either Withdrawn from the system, added to the bottom product from the flash operation and returnedto the absorption zone',

' added to thefeed to the flash tank, or added to the feed vtothe rich solvent stripper, depending on ultimately desired composition and required heat balance.

The drawing is a, diagrammatic representation of one embodiment of the invention. Vaporized" hydrocarbon feed is introduced into the extraction column I6, through conduit I, at about the middle.

Dry furfural is introduced near the top of the column on the top tray through conduit I3. The dry furfural flows downwardly countercurrent to the uprising vaporized feed. Water is introduced into column I6 near the bottom through conduit I5 and throughy at least onev of conduits is provided/for adding water from an external source to replace any lost from the system. Ac-

cumulator 20 is provided withr avent line- 1' for venting any absorbed hydrocarbons retained .in the lean furfural and subsequently desorbed in fractionator I8`to accumulate in accumulatory 20. The vented hydrocarbons may be sent to disposal or may becompressed and added to the overhead product from stripper |11 The vent gases from this accumulator will consist primarily of the same. hydrocarbons as those in the overhead product from stripper I1.

In order to prevent the accumulation of contaminants in the furfural, such as heavy polymers of iurfuralfand hydrocarbon polymers,.ltr

may be necessary to subjectthe bottomproduct of stripper I1 lto a purication step (not shown).

' This may be done either batchwise or continuvously by any satisfactory means, for example, a ymethod such as that disclosed in thecopending application of Buell et al. Serial No.`460,004, filed Sept. 28, 1942. The thus purified furfural may then be fed to column I8 for drying.

Example As an example of the operation of my invention, an unsaturated C4 petroleum fraction, from a source not shown, containing about 3 parts propane, 63 parts butadiene, 13 parts butene-l, 5 parts isobutylene, 13 parts butenes-2, and 3 parts n-butane along with a trace of vinyl acetylene and C5 and heavier was charged to absorber 23, 24, and 25. Reboil heat is supplied to vthe column at the bottomy to remove the less selectively dissolved hydrocarbons. These hydrocarbons pass upwardly throughthe column and are removed through line 2. At least a portion of the overhead product is condensed and refluxed to the absorber (means not shown because conventional). The furiural-water mixture rich with the selectively absorbed hydrocarbons leaves column I6 as the bottom product through conduit 3 to enter stripping column I1. Reboil heat is supplied at the bottom of the stripping column to I6 through line I located near the middle of absorber I6.

Lean substantially dry furfural from fractionator I8 was introduced into absorber I6 through line I3, near the top ofthe. absorber. Enough liquid water from accumulator 20 was introduced into absorber I6 near the 'bottom by means of lines 9 and I5 to give a resultant furfural-water mixture containing *abouty 5 per cent by weight of water. vRich furfural-water mixture containing 4 parts butadiene and 0.3 parts of butenes-2 alongwlth a trace of Ct and heavier and vinyl acetylene was withdrawn from the bottom of absorber I8 through line 3 and charged to stripture of'80 F. and bottom temperature of 240 F. Stripper I1 was operated at a pressure of 65 pounds per square inch absolute, reflux ratio 1:1, top temperature of 103 F. and bottom temperature of 300 F. Lean furfural-water mixture containing about 0.1 per drawn from the bottom of the stripper through line 5 and charged to fractionator I8. The stripper overhead product, consisting of 93 parts butadiene and '7 parts of butenes-2 along with traces of isobutylene, butene-l, C5 acetylene, and furfural, was charged to a fractionator (not shown) through lines 4 and 8 for pure butadiene.

fural as a bottom product and furfural-water azeotrope as the top product. The furfuralwater azeotrope was removed overhead from fractionator I8 through line 6 into condenser I9and the resulting condensate introduced as a liquid into accumulator where two liquid phases were allowed to separate. The bottom layer, the furfural-rich phase, was totally refluxed to column I8 by means of line I 0. The top layer, waterrich phase, was recycled to absorber I6 by means of lines 9 and I5 to supply the water required near the bottom of absorber I6. Accumulator 28 was also equipped with vent line 'I for venting absorbed hydrocarbons not completely stripped from the furfural-water solvent in stripper I'I. 'I'his vent stream was compressed to substantially I6. Fractionator I8 Was operated at essentially atmospheric pressure, top temperature of 208 F. and bottom temperature of 323 F.

It is to be understood that instead of water in the liquid form I may introduce steam as the source of water to the absorber I6. This steam may conveniently be formed by vaporizing the water recycle from accumulator 20 by passing it via line 2Ivthrough vaporizer '22 lwhence the steam thus formed is introduced into the absorber I6 via. line I5 and preferably via line 23 into the reboiler section thereof. Makeup steam from an extraneous source may be simpplied for this purpose via line 26.

Use of steam in preference to Water is advantageous in that introduction of steam supplies not only 4the desired proportion of water to reduce introducing steam which supplies the needed water and at least a part of the heat required in the absorber I6.

As used herein and in the appended claims the water is intended to designate either liquid water or steam unless otherwise specified.

Reference is made under Rule 43 to my prior and copending application, Serial No. 438,844. ied April 13, 1942, which discloses and claims the use of furfural containing a minor proportion of dissolved water in the extraction of aliphatic unsaturated hydrocarbons. y

I claim: 1. In the process of concentrating an aliphatic unsaturated hydrocarbon from a hydrocarbon Water is introduced near the top of said column and serves as selective solvent for the unsatmaintaining the water content of the furfural introduced as selective solvent into the top of the absorption column at sufficiently low level to prevent separation of two liquid phases in the upper portion of the absorption column, introducing bons is maintained in the furfural 1n the upper portion of the column while the boiling point of the'furfural in the lower portion of the column is lowered. o

2. The process of claim 1 carbon mixture is a mixture drocarbons.

3. The process of claim 1 wherein the hydrocarbon mixture is a mixture of aliphatic C4 hydrocarbons and wherein the unsaturated hydrocarbon is butene. f

4. The process of claim 1 wherein the hydrocarbon mixture is a mixture of aliphatic C4 hydrocarbons and wherein the unsaturated hydrocarbon is butadiene.

5. The process of claim 1 wherein said water is in the form of steam and is introduced in the reboiler section of said column.

KARL H. HACHMUTH.

wherein the hydroof aliphatic C4 hy- 

